Omnidirectional components of a dynamic microphone are controlled by resistance (resistance control). The dynamic microphone is therefore provided with an acoustic resistor disposed immediately behind a diaphragm to achieve flat frequency response.
FIGS. 7 and 8 illustrate a typical conventional dynamic microphone unit 200. As illustrated in FIGS. 7 and 8, a unit case 1 functions as a base of the microphone unit. The unit case 1 is a cylinder having a bottom surface. The unit case 1 has an inner cylinder 11 integrated thereto and extends from the top toward the bottom. A round flange 12 extends from the bottom of the inner cylinder 11 of the unit case 1 inward in the radial direction.
The inner cylinder 11 of the unit case 1 accommodates a magnetic circuit composed of the following magnetic circuit components. A dish-shaped yoke 2 fixed into the inner cylinder 11 is supported by the flange 12 of the inner cylinder 11. The outer surface of the circumferential wall of the yoke 2 is in contact with the inner circumferential surface of the inner cylinder 11. A disk magnet 3 fixed on the bottom plate of the yoke 2 has a smaller outer diameter than the inner diameter of the circumferential wall of the yoke 2. A disk pole piece 4 is fixed on the magnet 3. A ring yoke 21 is fixed on the top surface of the circumferential wall of the yoke 2. The pole piece 4 has substantially the same thickness as that of the ring yoke 21. The pole piece 4 and the ring yoke 21 are fixed so as to be substantially flush with each other. The outer circumferential surface of the pole piece 4 faces the inner circumferential surface of the ring yoke 21 with a proper gap to define a round magnetic gap. Most of these magnetic circuit components are contained in the inner cylinder 11. The top surface of the pole piece 4 is substantially flush with the top surface of the inner cylinder 11.
A magnetic flux from the magnet 3 returns to the magnet 3 through a magnetic circuit composed of the yoke 2, the ring yoke 21, the magnetic gap, and the pole piece 4. In other words, the magnetic flux traverses the magnetic gap. The magnet 3 has a smaller outer diameter than the outer diameter of the pole piece 4. An air chamber 9 having a larger width than that of the magnetic gap is defined between the outer circumferential surface of the magnet 3 and the inner circumferential surface of the ring yoke 21 below the magnetic gap. The yoke 2 has multiple through holes 22 at the bottom portion. The holes 22 connect the air chamber 9 to a space surrounded by the round flange 12 of the unit case 1.
The unit case 1 has a projection edge 14 along the outer circumference at the top. The unit case also has a concentric projection 13 inside the projection edge 14, the projection 13 having a height lower than the projection edge 14 on the top of the unit case 1. The circumferential edge of a diaphragm 5 is fixed on the top surface of the projection 13. The diaphragm 5 is a thin film composed of a material such as synthetic resin or metal. The diaphragm 5 includes a center dome 51 and a sub-dome 52 surrounding the center dome 51. The center dome 51 is a partial spherical shell. The sub-dome 52 has an arc-shaped cross section and extends along the circumferential edge of the center dome 51. The diaphragm 5 is fixed at its outer circumferential edge of the sub-dome 52, on the top surface of the projection 13. This enables the diaphragm 5 to vibrate in response to the sound pressure from received sound waves, in the anteroposterior direction (the vertical direction in FIG. 7) around the outer circumferential edge of the sub-dome 52 as a supporting node.
A voice coil 6 is fixed along a round boundary line between the center dome 51 and the sub-dome 52 in the diaphragm 5. The voice coil 6 is formed by winding a thin conductive wire and by fixing it into a cylindrical shape. One end of the cylindrical voice coil 6 is fixed to the diaphragm 5. The voice coil 6 is disposed in the magnetic gap while the outer circumferential edge of the sub-dome 52 in the diaphragm 5 is fixed to the projection 13 as described above. In this state, the voice coil 6 is separated from both the ring yoke 21 and the pole piece 4.
Near the obverse of the diaphragm 5, an equalizer 8 functioning also as a protector for the diaphragm 5 is fixed, at its circumferential edge, to the projection edge 14 of the unit case 1. The equalizer 8 has a ceiling surface having a dome shape in the center. A gap with a predetermined distance is defined between the ceiling surface and the center dome 51 of the diaphragm 5. The equalizer 8 has multiple holes 82 for introducing sound waves from the exterior to the diaphragm 5.
The bottom of the unit case 1 is closed to provide a relatively large air chamber 15 in the unit case 1. In the air chamber 15, an acoustic resistor 16 adheres to the bottom surface of the yoke 2. The flange 12 of the unit case 1 has a cylindrical inner surface. The inner surface of the flange 12 supports the outer circumferential of the acoustic resistor 16. The acoustic resistor 16 is composed of, for example, a thick-ply unwoven fabric. The acoustic resistor 16 is disposed adjacent to the reverse of the diaphragm 5. A space adjacent to the reverse of the diaphragm 5 is in communication with the acoustic resistor 16 through the magnetic gap, the air chamber 9, and the holes 22 of the yoke 2. The space adjacent to the reverse of the diaphragm 5 is also in communication with the air chamber 15.
The diaphragm 5 vibrates in the anteroposterior direction in response to a variation in the sound pressure from received sound waves. The voice coil 6 also vibrates in the anteroposterior direction in cooperation with the diaphragm 5. The voice coil 6 vibrates to traverse the magnetic flux passing through the magnetic gap. The voice coil 6 traverses the magnetic flux to generate electric power as audio signals in response to a variation in the sound pressure. A dynamic microphone unit 200 electro-acoustically converts the signals as described above. For example, audio signals are outputted from both ends of the voice coil 6 wired along the reverse of the sub-dome 52 to the exterior.
In such a configuration of the dynamic microphone unit 200, the space adjacent to the reverse of the diaphragm 5 is partitioned by the voice coil 6 into a space adjacent to the reverse of the center dome 51 and another space adjacent to the reverse of the sub-dome 52. In the dynamic microphone unit 200, these spaces are in communication with each other through magnetic gaps adjacent to the inner circumferential surface and adjacent to the outer circumferential surface of the voice coil 6. The sensitivity of the dynamic microphone unit 200 can be effectively improved by decreasing the widths of the magnetic gaps. In the dynamic microphone unit 200, the widths of the magnetic gaps are therefore as decreased as possible provided that the voice coil 6 does not come into contact with the pole piece 4 and the ring yoke 21. As a result, the space adjacent to the reverse of the diaphragm 5 is substantially partitioned by the voice coil 6 into the spaces adjacent to the reverse of the center dome 51 and adjacent to the reverse of the sub-dome 52, as described above.
The acoustic capacitance of the space adjacent to the reverse of the center dome 51 is referred to as Sc, and the acoustic capacitance of the space adjacent to the reverse of the sub-dome 52 to as Ss. The acoustic mass and the acoustic resistance of a gap between the inner circumferential surface of the voice coil 6 and the outer circumferential surface of the pole piece 4 are referred to as mgi and rgi, respectively. The acoustic mass and the acoustic resistance of a gap between the outer circumferential surface of the voice coil 6 and the inner circumferential surface of the ring yoke 21 are referred to as mgo and rgo, respectively. Sound pressure applied to the obverse of the diaphragm 5 is referred to as P1, the acoustic resistance of the acoustic resistor 16 disposed in the air chamber 15 of the unit case 1 to as r1, the acoustic mass of the air chamber adjacent to the obverse of the diaphragm 5 to as mo, and the acoustic capacitance of the adjacent air chamber to as So. The acoustic capacitance of the air chamber 9 between the inner surface of the circumferential wall of the yoke 2 and the outer circumferential surface of the magnet 3 is referred to as Sg. Accordingly, the acoustic capacitance Sc of the space adjacent to the reverse of the center dome 51 is connected to the acoustic capacitance Ss of the space adjacent to the reverse of the sub-dome 52 through the acoustic mass mgi, the acoustic resistance rgi, the acoustic capacitance Sg, the acoustic mass mgo, and the acoustic resistance rgo, as illustrated in FIG. 8.
FIG. 9 illustrates an equivalent circuit of the dynamic microphone unit 200 including the acoustic mass, the acoustic capacitance, and the acoustic resistance illustrated in FIGS. 7 and 8. The equivalent circuit of the dynamic microphone unit 200 illustrated in FIG. 9 includes the sound pressure P1, the acoustic mass mo, the acoustic capacitance So, the acoustic mass mgi, the acoustic resistance rgi, the acoustic resistance rgo, the acoustic mass mgo, and the acoustic capacitance Ss, which are connected in series. In the equivalent circuit of the dynamic microphone unit 200, connection nodes between the acoustic capacitance So and the acoustic mass mgi and between the sound pressure P1 and the acoustic capacitance Ss are connected to the acoustic capacitance Sc. In the equivalent circuit of the dynamic microphone unit 200, connection nodes between the acoustic resistance rgi and the acoustic resistance rgo and between the sound pressure P1 and the acoustic capacitance Ss are connected to the acoustic resistance r1 and the acoustic capacitance S1 connected in series. In the equivalent circuit of the dynamic microphone unit 200, the acoustic capacitance Sg is connected in parallel to the acoustic resistance r1 and the acoustic capacitance S1 connected in series.
As is apparent from the equivalent circuit of the dynamic microphone unit 200 in FIG. 9, a resonant circuit is defined by the acoustic mass mgi adjacent to the inner circumference of the magnetic gap partitioned by the voice coil 6 and the acoustic capacitance Sg of the air chamber 9. In the equivalent circuit of the dynamic microphone unit 200, another resonant circuit is defined by the acoustic mass mgo adjacent to the outer surface of the magnetic gap and the acoustic capacitance Ss of a space adjacent to the reverse of the sub-dome 52. The air chamber 9 has a small volume in comparison with that of the air chamber 15 occupying the lower half of the unit case 1. As is apparent from the equivalent circuit of the dynamic microphone unit 200, the acoustic capacitance Sg and the acoustic mass mgi readily resonate in cooperation. The resonance causes a peak in a specific frequency in the dynamic microphone unit 200 and leads to improper frequency characteristics.
In order to decrease the resonance, the volume of the air chamber 9 may be further decreased to minimize the acoustic capacitance Sg to a negligible level to prevent the acoustic capacitance Sg from resonating with the acoustic mass mgi. FIG. 10 illustrates such a configuration of a typical conventional dynamic microphone unit 300. In the dynamic microphone unit 300, an acoustic resistor 25 is disposed in an air chamber 9 between the inner circumferential surface of the circumferential wall of a yoke 2 and the outer circumferential surface of a magnet 3. In the dynamic microphone unit 300, the acoustic resistor 25 is shifted to so as to be in contact with the bottom surface of the yoke 2. In the dynamic microphone unit 300, an air chamber 9 is provided above the top surface of the acoustic resistor 25. This causes the acoustic resistor 25 to limit the volume of the air chamber 9, resulting in a significantly small acoustic capacitance Sg of the air chamber 9. The acoustic capacitance Sg is connected to an acoustic capacitance S1 through the acoustic resistance r1 of the acoustic resistor 25 and holes 22 of the yoke 2. FIG. 11 illustrates an equivalent circuit of the dynamic microphone unit 300. In the equivalent circuit of the dynamic microphone unit 300 illustrated in FIG. 11, the acoustic capacitance Sg is limited to a negligible level, i.e., a significantly small acoustic capacitance by the existence of the acoustic resistor 25, as described with reference to FIG. 10. The acoustic capacitance Sg is therefore omitted in FIG. 11. As described above, the dynamic microphone unit 300 illustrated in FIG. 10 can prevent resonance caused by the air chamber 9 to provide proper frequency characteristics not having a peak in an audible frequency band.
In order to dispose the acoustic resistor 25 in the air chamber 9 behind a voice coil 6 as described above to significantly decrease the volume of the air chamber 9, the acoustic resistor 25 must be disposed near the voice coil 6. If the acoustic resistor 25 is composed of a felt material, unwoven fabric, or an unwoven fabric material, fibers 251 of the acoustic resistor 25 partly rises as illustrated in FIG. 10. If the diaphragm 5 vibrates exceedingly, the voice coil 6 comes into contact with the fibers 251 to generate abnormal noise. In addition, the fibers 251 disturb vibration of the voice coil 6 in accurate response to sound waves to cause inaccurate electro-acoustic conversion. The dynamic microphone unit 300 therefore has a limited reduction in a distance between the acoustic resistor 25 and the voice coil 6. The dynamic microphone unit 300 also has a limited reduction in the acoustic capacitance Sg by a decrease in the volume of the air chamber 9. Even the dynamic microphone unit 300 having such a configuration therefore can prevent limited resonance caused by the air chamber 9.
The present inventor proposed a dynamic microphone including a voice coil having lead wires along the inner surface of a sub-dome of a diaphragm facing a ring yoke, and a magnetism generator circuit provided with amplitude restriction means that restricts the maximum displacement of vibration of the diaphragm toward a pole piece to a position at which the lead wires do not come into contact with the ring yoke (see Japanese Unexamined Patent Application Publication No. 2005-260306).
The present inventor also proposed a dynamic microphone including a voice coil having lead wires elastically held on a sub-dome through an elastic layer painted on the inner surface of the sub-dome of a diaphragm adjacent to the voice coil, so as not to break the lead wires even if the diaphragm is biased against a magnetism generator circuit (Japanese Unexamined Patent Application Publication No. 2006-019791).